Method of recording holographic information

ABSTRACT

A method of recording holographic information includes recording a hologram mark and a homogeneous mark in a holographic data storage medium with a volume to alternatively locate the hologram mark and the homogeneous mark. The hologram mark has a varied refractive index distribution due to constructive/destructive interferences between two light beams and indicates information, while the homogeneous mark has a more uniform refractive index distribution than the hologram mark.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.10-2008-0073049, filed on Jul. 25, 2008, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a method of recordingholographic information, and more particularly, to a method of recordingholographic information that ensures uniform recording quality of a markand inhibits the occurrence of aberrations in recording/reproductionspots during recording of data on a plurality of recording layers toimprove signal quality.

2. Description of the Related Art

In recent years, data storage technology using holograms has attractedconsiderable attention. A method of storing data using hologramsincludes storing data in the shape of optical interference fringes ininorganic crystals or in polymer materials that are sensitive to light.An optical interference fringe is formed by two laser beams withcoherency. That is, a reference beam and a signal beam travelling alongdifferent paths interfere with each other to form an interference fringethat causes chemical or physical changes to a photosensitive storagemedium, thus recording data in the photosensitive storage medium. Inorder to reproduce data from the recorded interference pattern, areference beam similar to the reference beam used for recording the datais irradiated to the interference pattern recorded in the photosensitivestorage medium. As a result, diffraction occurs due to the interferencepattern, so that the signal beam is restored to reproduce the data.

The above-described hologram data storage technology includes a volumeholography methods in which data is recorded and reproduced in pageunits using volume holography and a micro-holography method in whichdata is recorded and reproduced bit by bit using micro-holography. Inthe volume holography method, a vast amount of data may be processed atthe same time, but it is difficult to make data storage devices based onthis method commercially available since an optical system based on thevolume holography method requires very fine adjustments.

In the micro-holography method, two condensed beams are directed tointerfere with each other at a focus to form a fine interference fringe.Data storage is accomplished by moving the fine interference fringe on aplane of a data storage medium so that a large amount of data isrecorded, thereby forming a recording layer. A plurality of suchrecording layers may be formed in a depth-wise direction of the datastorage medium by superposition, thereby three-dimensionally recordingdata on the data storage medium.

In order to increase recording capacity in the micro-holography method,a plurality of recording layers may be formed or data may be recorded byoverlapping a plurality of wavelengths using wavelength-selectivereflection characteristics of a hologram.

However, during a recording process, the sensitivity of a photosensitivedata storage medium in which a hologram is recorded (hereinafter,referred to as “holographic data storage medium”) gradually decreases.Specifically, the sensitivity of the holographic data storage medium toincident energy is reduced during the recording process so that even ifdata is recorded with the same energy, the diffraction efficiency of thehologram may be less than at the beginning. Accordingly, when data isrecorded with the same energy, a portion where at least two holographsoverlap one another has reduced diffraction efficiency. Therefore, aportion where a previously recorded mark overlaps an additionallyrecorded mark may have a varied reflection rate in a multi-wavelengthrecording mode.

Also, when data is recorded to form a plurality of recording layers, aplurality of hologram marks are present in a space through whichrecording/reproduction beams travel. A portion of the holographic datastorage medium where the hologram mark is formed has a differentrefractive index from nearby portions. Since the phase of light directedat a particular position of the holographic data storage mediumincluding a hologram mark is altered due to the difference in therefractive index of the hologram mark compared to surrounding regions, awave front of the light is varied by as much as a non-uniformity in arefractive index distribution. Accordingly, when data is recorded on orreproduced from a recording layer disposed far from the surface of theholographic data storage medium, a wave front of light traveling throughan already recorded hologram mark is distorted, and the wave frontdistortion (or wave front aberration) affects the recording beams. Anincrease in wave front aberrations may lead to deformation of opticalspots that converge into a focus, thus increasing the size of theoptical spots. The increase in the size of the optical spots may resultin deterioration of formation of a recording mark in a recording modeand quality degradation of a reproduction signal in a reproduction mode.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a method of recordingholographic information using a micro-holographic datarecording/reproduction apparatus capable of recording a plurality ofrecording marks in a uniform medium having a volume. The method mayensure a uniform recording quality of marks during wavelengthmultiplexing and inhibit occurrence of aberrations inrecording/reproduction spots during recording of data on a plurality ofrecording layers to improve signal quality.

According to an embodiment of the present invention, there is provided amethod of recording holographic information, which includes recording ahologram mark and a homogeneous mark in a holographic data storagemedium having a volume to alternatively locate the hologram mark and thehomogeneous mark. The hologram mark has a varied refractive indexdistribution due to constructive/destructive interferences between twolight beams and indicates information, while the homogeneous mark has amore uniform refractive index distribution than the hologram mark.

According to an aspect of the present invention, a thickness orrefractive index variation of the homogeneous mark may be controlledsuch that a phase shift of a light beam passing through the homogeneousmark is about the same as a phase shift of a light beam passing throughthe hologram mark.

According to an aspect of the present invention, the homogeneous markmay be recorded by a first light source and the hologram mark may berecorded by a second light source.

According to an aspect of the present invention, after one of two lightbeams used for recording the hologram mark is cut off, the homogeneousmark may be recorded using the remaining light beam.

According to an aspect of the present invention, the homogeneous markmay be recorded by changing a focal position of one of two light beamsused for recording the hologram mark to reduce formation of aninterference fringe.

According to an aspect of the present invention, the hologram mark andthe homogeneous mark may be recorded using the same light source whilevarying coherence by changing driving conditions.

According to an aspect of the present invention, the hologram mark andthe homogeneous mark may be alternately recorded on all recordinglayers.

According to an aspect of the present invention, only a hologram markmay be recorded in at least one recording layer that affects less to anadjacent recording layer out of all recording layers, while a hologrammark and a homogeneous mark may be alternately recorded on the remainingrecording layers.

According to another embodiment of the present invention, there isprovided a method of recording holographic information, includingirradiating a holographic data storage medium with a first light beamand a second light beam according to data to be recorded, such that aplurality of hologram marks each having a varied refractive indexdistribution due to constructive/destructive interferences between thefirst light beam and the second light beam are formed in the holographicdata storage medium, and forming a plurality of homogeneous marksinterspersed between the plurality of hologram marks, each homogeneousmark having a refractive index distribution that is more uniform thanthe refractive index distribution of the hologram marks.

According to another embodiment of the present invention, there isprovided a holographic data recording/reproducing apparatus that recordshologram marks and homogenous marks on a holographic data storagemedium, the holographic data recording/reproducing apparatus includingan optical pickup unit including a light source, an optical pathseparator that divides light from the light source into a first lightbeam and a second light beam, a first light path and a second light pathalong which the first light beam and the second light beam,respectively, are directed to be incident on the holographic datastorage medium, a first focus variation unit and a second focusvariation unit that selectively focus the first light beam and thesecond light beam, respectively, such that hologram marks each having avaried refractive index distribution due to constructive/destructiveinterferences between the first light beam and the second light beam areselectively formed on the hologram data storage medium; and a shutterthat selectively blocks the first light path; and a controller thatcontrols one of the light source, the first focus variation unit and theshutter such that homogeneous marks each having a refractive indexdistribution that is more uniform than the refractive index distributionof the hologram mark are selectively formed on the hologram data storagemedium.

According to another aspect of the present invention, there is provideda holographic data recording/reproducing apparatus that records hologrammarks and homogenous marks on a holographic data storage medium, theholographic data recording/reproducing apparatus including an opticalpickup unit including a first light source, a first optical pathseparator that divides light from the first light source into a firstlight beam and a second light beam, a first light path and a secondlight path along which the first light beam and the second light beam,respectively, are directed to be incident on the holographic datastorage medium, a first focus variation unit and a second focusvariation unit that selectively focus the first light beam and thesecond light beam, respectively, such that hologram marks each having avaried refractive index distribution due to constructive/destructiveinterferences between the first light beam and the second light beam areselectively formed on the hologram data storage medium; a second lightsource; and a second optical path separator that directs light from thesecond light source to travel through the first light path or the secondlight path such that a homogeneous mark is formed on the hologram datastorage medium.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 is a graph showing variations in cumulative grating strength (M#)and recording sensitivity of a holographic data storage medium accordingto cumulative energy provided during recording;

FIG. 2 is a diagram for explaining a variation in reflection rate of anadditional recording mark due to overlapping of marks in amulti-wavelength recording;

FIGS. 3A through 3D are schematic diagrams illustrating a method ofrecording micro-holographic information using a shutter according to anexemplary embodiment of the present invention;

FIG. 4 is a diagram illustrating a process of forming a recording markaccording to a existing method of recording holographic information,wherein a light beam passing through a region between hologram marks hasa different phase shift from a light beam passing through the hologrammark;

FIG. 5 is a diagram illustrating a process of forming a recording markaccording to a method of recording holographic information according toan exemplary embodiment of the present invention, wherein a homogeneousmark is formed between hologram marks and a light beam passing throughthe homogeneous mark has about the same phase shift as a light beampassing through the hologram mark;

FIG. 6 is a schematic diagram showing an example of an opticalconstruction of a holographic data recording/reproduction apparatuscapable of embodying a method of recording holographic informationaccording to an exemplary embodiment of the present invention;

FIGS. 7A and 7B are schematic diagrams illustrating a method ofrecording holographic information using a focus adjuster according to anexemplary embodiment of the present invention;

FIGS. 8A and 8B show a lasing spectrum and the visibility of aninterference fringe, respectively, when a semiconductor laser diode (LD)oscillates in a single mode;

FIGS. 9A and 9B show a lasing spectrum and the visibility of aninterference fringe, respectively, when a semiconductor LD oscillates ina multi-mode;

FIGS. 10A and 10B are schematic diagrams illustrating a method ofrecording holographic information according to an exemplary embodimentof the present invention when coherence is varied by changing drivingconditions of a light source;

FIG. 11 is a diagram showing another example of an optical constructionof a holographic data recording/reproduction apparatus capable ofembodying a method of recording holographic information according to anexemplary embodiment of the present invention; and

FIGS. 12A and 12B are schematic diagrams illustrating a method ofrecording holographic information using two independent light sourcesaccording to an exemplary embodiment of the present invention.

FIG. 13 is a schematic diagram showing the holographic datarecording/reproducing controller, light source, first focus variationunit and shutter according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

According to existing multilayered optical recording technology,respective recording layers are stacked on a data storage medium so thatthe recording capacity of the data storage medium may be increased byrecording data using optical absorption of the recording layers.However, this method involves a very complicated process of stacking aplurality of recording layers and leads to a reduction in yield, therebyincreasing the production cost of the data storage medium.

A micro-holographic recording method in which a plurality of virtuallayers are stacked in a uniform recording material has been proposed asa new optical recording technique capable of recording/reproducinghigh-capacity data at lower cost than existing optical recordingtechniques. According to the micro-holographic recording method, anactual recording layer is not present in a recording medium, but arecording mark is recorded parallel to the surface of the recordingmedium at an arbitrary height. Then, a recording operation is repeatedby changing the height at which the recording mark is recorded, so thata plurality of recording layers on which data is recorded can be stackedso as to increase recording capacity.

The recording medium used in the micro-holographic recording method maybe formed of a uniform medium. At least one reference surface may beformed on the recording medium to provide a reference in the heightdirection of the recording layer. Foci of two beams traveling inopposite directions are formed in the recording medium using beamsincident to one side or both sides of the recording medium at a desiredheight, determined using of the reference surface, and interfere witheach other to provide a fine interference fringe recorded as a hologrammark.

The hologram mark is recorded on a virtual surface parallel to (or withthe same height to) the surface of the recording medium. The recordedvirtual surface has a similar shape as a recording layer of a datastorage medium used in a conventional optical recording technique. Afterone recording layer is recorded, a new recording layer is recorded byadjusting the height of an optical focus appropriately. An intervalbetween recording layers may vary depending on a recording method, thesize of optical foci, and a method of measuring signals. In order toachieve high recording density, the interval between recording layersmay range from several μm to several tens of μm. A plurality ofrecording layers may be stacked by repeating the above-describedprocess, so it is possible to increase recording capacity. Also, inorder to increase recording capacity, data may be recorded byoverlapping a plurality of hologram marks formed at differentwavelengths using the wavelength-selective reflection characteristics ofa hologram.

However, the sensitivity of a holographic data storage medium decreasesduring recording due to the reaction of the holographic data storagemedium during recording. In other words, the reaction rate of theholographic data storage medium to incident energy decreases asrecording progresses so that even if data is recorded with the sameenergy, the diffraction efficiency of the hologram becomes less, asshown in FIG. 1.

FIG. 1 is a graph showing variations in cumulative grating strength (M#)(-♦-) and recording sensitivity (--) of a holographic data storagemedium during recording.

Referring to FIG. 1, it can be seen that when incident energy isaccumulates during a recording proceeding, the recording sensitivity ofthe holographic data storage medium sharply decreases, and thecumulative grating strength M# thereof quickly reaches a limit. Thecumulative grating strength M#, which is the sum of square roots ofdiffraction efficiencies of the holographic data storage medium, is anindex of recording capacity of the holographic data storage medium. Thesensitivity of the holographic data storage medium corresponds to anincrement in the cumulative grating strength M# relative to incidentenergy.

As stated above, the diffraction efficiency of the holographic datastorage medium drastically decreases even when the same energy is usedthroughout the recording process. Thus, when data is recorded with thesame energy, the reflectivity at a portion where two holograms overlapeach other becomes less efficient.

For example, when data is recorded on the same layer using 405nm-wavelength light and 450 nm-wavelength light, a portion whosereflection rate is varied due to the overlapping of the marks may begenerated as shown in FIG. 2.

Referring to FIG. 2, a portion of a left mark out of marks recordedusing the 450 nm-wavelength light overlaps a mark recorded using the 405nm-wavelength light. As can be seen from a graph of FIG. 2, even if datais recorded with the same energy, the reflection rate of the overlappedportion is less than that of an non-overlapped portion. Therefore,performing a multi-wavelength recording operation on a single layer maybe difficult due to a difference in reflection rate between theoverlapped portion and the non-overlapped portion.

Although there is a method of recording data by spacing layers accordingto wavelengths, this method requires an additional driver unit capableof precisely controlling layers according to wavelength to maintain thelayers at an interval of, for example, about 1˜2 μm.

When data is recorded on a plurality of recording layers, a plurality ofhologram marks are present in a space through whichrecording/reproduction beams travel. A portion of the holographic datastorage medium where the hologram mark is formed has a differentrefractive index from nearby portions. Since the phase of light directedat a particular position of the holographic data storage medium thatincludes a hologram mark is altered due to the difference in therefractive index of the hologram mark compared to that of surroundingregions, a wave front of the light is varied by as much as anon-uniformity in a refractive index distribution.

Accordingly, when data is recorded on or reproduced from a recordinglayer disposed far from the surface of the holographic data storagemedium, a wave front of light traveling through an already recordedhologram mark is distorted, and the wave front distortion (or wave frontaberration) affects recording beams. An increase in wave frontaberrations may lead to deformation and an increase in size of opticalspots that converge into a focus. The increase in the size of theoptical spots may result in a deterioration of the formation of arecording mark in a recording mode and a quality degradation of areproduction signal in a reproduction mode. In a micro-holographymethod, the Strehl ratio (SR) is known to be approximately expressed asin Equation 1. The Strehl ratio(SR) is a ratio of distortion of an idealGaussian beam due to aberrations.

SR=1−M _(η)  (1),

wherein M denotes the number of layers through which incident lightpasses, and η denotes the diffraction efficiency of each hologram mark.The aberrations are equally applied to two beams (i.e., a signal beamand a reference beam) that form a focus.

For example, assuming that twenty recording layers are recorded and thereflection rate of each hologram mark is 1%, when the uppermost layer isfinally recorded, the Strehl ratio (SR) of incident light travelingupward from below a holographic data storage medium far from an initialincidence surface of the holographic data storage medium is only 0.6,which corresponds to an aberration of about 100 mλ greater than a knownaberration limit (about 70 mλ) of a typical optical recording technique.

Accordingly, when the last layer is finally recorded, recordingcharacteristics may be seriously degraded due to the aberrations. Forthe same reason, when the lowermost layer is reproduced, the aberrationreaches about 70 mλ, thus bringing about a marginless system. Thisproblem becomes more serious as the number of recording layersincreases.

By simple calculation, when a system with a hologram-mark diffractionefficiency of about 1% includes more than twenty recording layers, evenif the system is near perfect, reproduction of the lowermost layer mayresult in signal degradation.

According to a method of recording holographic information according toan exemplary embodiment, a micro-holographic data recording/reproductionapparatus capable of recording a plurality of layers in a medium with avolume may ensure a uniform recording quality of a mark duringwavelength multiplexing and inhibit occurrence of aberration inrecording/reproduction spots in a multilayered recording mode so as toimprove signal quality. Specifically, a method of recording holographicinformation according to an embodiment of the present invention includesrecording a homogeneous mark (i.e., a non-fringe mark) with no or fewinterference fringes and a more uniform refractive index than a hologrammark in a portion of the holographic data storage medium apart from thehologram mark.

As explained above, in a micro-holography technique of forming aplurality of recording layers on a uniform holographic data storagemedium to achieve high recording/reproduction density, two incidentlight beams may be used (or a light beam already used to form a focus isreused) and incident to one side or both sides of the holographic datastorage medium.

Two light beams are directed to interfere with each other at a focus toform an interference fringe, and the refractive index of the holographicdata storage medium is periodically varied according to the interferencefringe to enable information recording. An interference fringe is notrecorded in a portion of the holographic data storage medium having nomark. Optical systems used for recording/reproducing micro-holographicinformation may be divided into categories of a one-sided incidenceoptical system in which light is incident to one side of a holographicdata storage medium and a two-sided incidence optical system in whichlight is incident to both sides of a holographic data storage medium. Inaddition, an optical system using a retro-reflector may reflect lightpassing through a medium to record/reproduce micro-holographicinformation.

In the micro-holography technique, since a recording mark is basicallyformed using a variation in refractive index of a medium, aberration mayoccur in incident light due to a hologram mark.

A method of recording holographic information according to an exemplaryembodiment of the present invention includes reducing aberrations. Basicprinciples of the method will be described with reference to FIGS. 3Athrough 3D.

FIGS. 3A through 3D are schematic diagrams illustrating a method ofrecording micro-holographic information according to an exemplaryembodiment of the present invention. Although FIGS. 3A through 3Dexemplarily illustrate that two light beams are incident to respectivesides of a holographic data storage medium, the method of recordingholographic information according to an embodiment of the presentinvention may be applied not only to a two-sided incidence holographicdata recording/reproduction apparatus but also to a one-sided incidenceholographic data recording/reproduction apparatus in which two lightbeams are incident to one side of a holographic data storage medium.

FIGS. 3A through 3D illustrate a variation in incident light over timeand formation of a recording mark in a recording mode. That is, FIGS. 3Athrough 3D illustrate a partial process of recording information bytime-controlling incident light according to rotation of a holographicdata storage medium 190. FIGS. 3A through 3D illustrate a case in whicha holographic data recording/reproduction apparatus of FIG. 6 isapplied. Thus, first and second objective lenses 160 and 170 will bedescribed later.

Information may be recorded in the order of, for example, FIGS. 3Athrough 3D. In FIGS. 3A through 3D, two kinds of recording marks areformed. One kind of recording mark is a hologram mark 10 a with the sameshape as a conventional micro-hologram mark. The hologram mark 10 a hasan interference fringe (or hologram) formed by two light beams travelingin opposite directions. Also, the hologram mark 10 a has a periodicallyvaried refractive index distribution and represents information (ordata). The other kind of recording mark is a homogeneous mark 10 bproviding a refractive index distribution that is more uniform than thatprovided by the hologram mark 10 a.

The hologram mark 10 a and the homogeneous mark 10 b may be formed invarious manners. FIGS. 3A through 3D exemplarily illustrate a method offorming the hologram mark 10 a and the homogeneous mark 10 b using ashutter 140. For clarity of explanation, an optical system in whichincident light beams are separately incident to respective sides of theholographic data storage medium 190 is illustrated in FIGS. 3A through3D.

Referring to FIGS. 3A and 3C, when two light beams are incident from alight source to respective sides of the holographic data storage medium190 and converge into a focus, an interference fringe is formed. Sincethe refractive index of the holographic data storage medium 190 variesin proportion to the intensity of the interference fringe, the hologrammark 10 a, which is illustrated with a fringe, is recorded in theholographic data storage medium 190 due to a periodic variation inrefractive index. The focus is located on a virtual recording layer onwhich the hologram mark 10 a is to be formed.

Referring to FIGS. 3B and 3D, after the hologram mark 10 a has beenformed and the medium 190 is rotated so that the focus is located wherea mark has not yet been formed, one of two incident light beams is cutoff by the shutter 140 such that only the remaining incident light beamis incident to the holographic data storage medium 190 and convergesinto a focus. The refractive index of the holographic data storagemedium 190 is altered due to the intensity of the incident light beam.In this case, since no interference fringe is formed, the refractiveindex of the holographic data storage medium 190 is varied onlyaccording to optical intensity to record a homogeneous mark 10 b. FIGS.3B and 3D illustrate uniform variations in refractive index, which maybe obtained in the holographic data storage medium 190 with a criticalreaction value. Here, the holographic data storage medium 190 with thecritical reaction value may be formed of a material whose refractiveindex varies only at a predetermined energy or more.

Due to the drive of the shutter 140, the hologram mark 10 a withinterference fringes and the homogeneous mark 10 b with a uniformvariation in refractive index are alternately formed in the holographicdata storage medium 190.

The recording method shown in FIGS. 3A through 3D may be modified. Forexample, a recording layer may be formed in which hologram marks 10 aare recorded, and then, one light beam may be cut off using the shutter140, and the homogeneous marks 10 b may be recorded between the hologrammarks 10 a.

According to the method of recording holographic information accordingto the present exemplary embodiment, the homogeneous mark 10 b isrecorded in a region of a recording layer where the hologram mark 10 ais not recorded.

Therefore, unlike a conventional micro-holography method, which leaves avacancy between hologram marks, according to the method of the presentexemplary embodiment, a homogeneous marks 10 b, with a uniform variationin refractive index, are provided between the hologram marks 10 a.

Since the holographic data storage medium 190 has a very low refractiveindex variation of about 0.01 or less, the homogeneous mark 10 b mayhave a very low reflection rate of about 1e⁻³% or less. Of course, sincea critical reaction of the holographic data storage medium 190 isimperfect, as the refractive index at boundaries slowly decreases, thereflection rate of the homogeneous mark 10 b in an optical axisdirection may further decrease.

On the other hand, since light is reflected by the hologram mark 10 adue to interference between a plurality of repetitive patterns, thehologram mark 10 a may have a much higher reflection rate than thehomogeneous mark 10 b. For example, when interference fringes have athickness of about 3 μm and a refractive index variation of the hologrammark 10 a has a maximum value of about 0.01, the hologram mark 10 a hasa reflection rate of about 5%. Thus, the homogeneous mark 10 b producesno or very little effect on a reproduction signal.

Meanwhile, recording conditions may be controlled in a recording modesuch that a light beam passing through the homogeneous mark 10 b hasabout the same phase shift as a light beam passing through the hologrammark 10 a. As a result, aberrations of incident beams generated duringpassing through the recording layer in recording can be removed.

FIG. 4 is a diagram illustrating a process of forming a recording markin which light beams pass through an already-recorded layer, accordingto a conventional method of recording holographic information. Referringto FIG. 4, it can be seen that a light beam {circle around (1)}′ passingthrough a region between hologram marks of an already-recorded layerwould have a different phase shift from a light beam {circle around(2)}′ passing through the hologram mark.

FIG. 5 is a diagram illustrating a process of forming a recording markin which light beams pass through an already-recorded layer, accordingto a method of recording holographic information according to anexemplary embodiment of the present invention. Referring to FIG. 5, itcan be seen that a homogeneous mark is formed between hologram marks sothat a light beam {circle around (1)} passing through the homogeneousmark of the already-recorded layer would have about the same phase shiftas a light beam {circle around (2)} passing through the hologram mark.

As can be seen from FIGS. 4 and 5, when data is recorded by controllingthe recording conditions to nearly equalize the phase shift of a lightbeam passing through a homogeneous mark with that of a light beampassing through a hologram mark of an already-recorded layer, avariation in phase according to the position of the light beam hardlyoccurs, thereby minimizing aberrations of incident light beams generatedduring passing through a recording layer.

Meanwhile, when the homogeneous mark 10 b is formed in a region wherethe hologram mark 10 a is not present, consumption of the holographicdata storage medium 190 in a region with the hologram mark 10 a issimilar to that of the holographic data storage medium 190 in the regionwithout the hologram mark 10 a. As a result, a non-uniformity in themark reflection rate can be eliminated unlike in a conventionalmulti-wavelength recording mode, thereby improving signal quality. Thatis, the homogeneous mark 10 b is formed between the hologram marks 10 ain a single recorded recording layer so that the entire recording layeris irradiated with energy at one time. Thus, when a recording layerrecorded using a light beam with one wavelength is then recorded using alight beam with another wavelength during multi-wavelength overlapping,all regions are irradiated with energy the same number of times.Therefore, a non-uniformity in mark reflection rate hardly occurs due tothe multi-wavelength overlapping, thereby improving signal quality.

In the method of recording holographic information according to theexemplary embodiment of the present invention, the homogeneous mark 10 bis not limited to the perfectly homogeneous mark shown in FIGS. 3A, 3B,and 5. The homogeneous mark may also include a mark in which there is nodifference between constructive and destructive portions of interferencefringes or a mark in which there is a smaller difference therebetweenthan the hologram mark 10 a such that the homogeneous mark has a verylow reflection rate. The thickness or refraction index variation of thehomogeneous mark 10 b may depend on the kind or state of the holographicdata storage medium 190. Basically, the thickness or refraction indexvariation of the homogeneous mark 10 b should be determined such thatthe homogeneous mark 10 b allows about the same consumption of theholographic data storage medium 190 as the hologram mark 10 a and alight beam passing through the homogeneous mark 10 b has about the samephase shift as a light beam passing through the hologram mark 10 a.

Since the hologram mark 10 a shows a periodic variation in refractiveindex due to constructive and destructive inferences, the homogeneousmark 10 b may be formed to have about the same thickness as the hologrammark 10 a and to have a refractive index that is the average of theperiodic variation of the hologram mark 10 a. Also, the homogeneous mark10 b may show a variation in refractive index corresponding toconstructive interference and have about half the thickness of thehologram mark 10 a.

FIGS. 3A through 3D further illustrate a method of recording holographicinformation using the shutter 140, which may be, for example, anacoustic-optic modulator, according to the exemplary embodiment of thepresent invention. The shutter 140 allows one of two incident lightbeams to pass therethrough or cuts off one of the two incident lightbeams. Thus, when one of the incident light beams passes through theshutter 140, the hologram mark 10 a is recorded, while when one of theincident light beams is cut off, the homogeneous mark 10 b is recorded.

In order to embody the method of recording holographic informationaccording to the exemplary embodiment, a focus controller may beintroduced instead of the shutter 140. Alternatively, while employinglight beams emitted from a single light source, coherence may be changedby modifying driving conditions of the light source. Alternatively, twoindependent light sources may be adopted.

Hereinafter, various examples of an optical construction of aholographic data recording/reproduction apparatus to which the method ofrecording holographic information according to an embodiment of thepresent invention can be applied will be explained.

FIG. 6 is a schematic diagram showing an example of an opticalconstruction of a holographic data recording/reproduction apparatus thatmay embody a method of recording holographic information according to anexemplary embodiment of the present invention.

Referring to FIG. 6, the holographic data recording/reproductionapparatus records data on a holographic data storage medium 190 whoseboth sides are irradiated with light beams, and reproduces recordeddata. The apparatus includes an optical pickup unit 100, whichirradiates light beams to the holographic data storage medium 190 andreceives the irradiated light beams, and a circuit unit (not shown).

The optical pickup unit 100 may include a light source 110, an opticalpath separator 130, a first objective lens 160, a second objective lens170, and a photodetector (PD) 180.

The light source 110 emits light L. The optical path separator 130branches the light L emitted by the light source 110 into a signal beamL1 and a reference beam L2. The first objective lens 160 condenses thesignal beam L1 onto the holographic data storage medium 190, while thesecond objective lens 170 condenses the reference beam L2 onto theholographic data storage medium 190. The photodetector 180 detects areproduction beam L2 r′ reflected from the holographic data storagemedium 190.

The optical pickup unit 100 may further include first and second focusvariation units 150 and 153, which vary a focal position of L1 and L2,respectively. In addition, the optical pickup unit 100 may furtherinclude a collimating lens 120, which collimates the light L emitted bythe light source 110, and first through third reflection members 132,134, and 136, which appropriately fold an optical path. Moreover, theoptical pickup unit 100 may further include a servo optical system (notshown), which performs a servo operation.

The light source 110 and the optical path separator 130 constitute alight source unit that emits recording/reproduction beams.

The light source 110 may be, for example, a semiconductor laser diode(LD) that emits blue light.

The collimating lens 120 may collimate the recording/reproduction lightL emitted by the light source 110. FIG. 6 exemplarily illustrates thatthe collimating lens 120 is interposed between the light source 110 anda polarization converter 125. As an alternative, the collimating lens120 may be located between the polarization converter 125 and theoptical path separator 130 or on other locations in the optical path.

Typically, the semiconductor that is used as the light source 110 mainlyemits a laser beam of one polarization element. In this case, thepolarization converter 125 may be located between the light source 110and the optical path separator 130.

The polarization converter 125 may be a wave plate, such as a half-waveplate or a quarter-wave plate. When an active half-wave plate is used asthe polarization converter 125, the wave plate rotates the polarizationdirection of linearly polarized light incident to the holographic datastorage medium 190 and may convert the linearly polarized light intolight with two orthogonal linearly polarization elements, that is, anS-polarization element and a P-polarization element. When an activequarter-wave plate is adopted as the polarization converter 125, it mayconvert predetermined incident linearly polarized light into circularlypolarized light. The circularly polarized light may be decomposed intotwo orthogonal linearly polarization elements. As described above, theS-polarization element and the P-polarization element traveling throughthe polarization converter 125 may be used as the signal beam L1 and thereference beam L2, respectively, during a recording.

The polarization converter 125 may be an active type that performs apolarization conversion function during a recording operation and doesnot perform the polarization conversion function during a reproductionoperation. That is, the polarization converter 125 may be an activehalf-wave plate or an active quarter-wave plate. When such activeelement is used as the polarization converter 125, almost all beamsemitted by the light source 110 may be used as a reproduction beamduring the reproduction operation.

The holographic data recording/reproduction apparatus embodies amicro-holography method in which an interference fringe formed byinterference between the signal beam L1 and the reference beam L2contains single-bit data for each focus. The light L emitted by thelight source 110 is modulated and emitted bit by bit. Accordingly, sinceboth the signal beam L1 and the reference beam L2 contain recordinginformation, there is no substantial difference in a recording processbetween the signal beam L1 and the reference beam L2, and the terms“signal and reference beams” may be used interchangeably. For brevity, alight beam traveling through the same optical path as a reproductionbeam L2 i′ incident to the holographic data storage medium 190 isdenoted by the reference beam L2.

The optical path separator 130 may branch the light L emitted by thelight source into two orthogonal polarization elements and allows therespective polarization elements to be irradiated to the holographicdata storage medium 190 along separate optical paths. The optical pathseparator 130 may be a polarization beam splitter, which transmits orreflects light depending on a polarization direction. For example, theoptical path separator 130 may directly transmit incident P-polarizedlight and reflect incident S-polarized light. In a reproduction mode,the optical path separator 130 may also separate the reproduction lightL2 i′ incident to the holographic data storage medium 190 from thereproduction beam L2 r′ reflected from the holographic data storagemedium 190.

The photodetector 180 may be located on one side of the optical pathseparator 130 to detect the reproduction beam L2 r′ that is reflectedfrom the holographic data storage medium 190 and passes through theoptical path separator 130.

The signal beam L1 and the reference beam L2, which are branched by theoptical path separator 130, may pass through a condensing optical systemand be incident on the holographic data storage medium 190.

The holographic data storage medium 190 may be a transmissive medium inwhich respective sides are irradiated with the signal beam L1 and thereference beam L2. In this case, the condensing optical system may bedivided into a first condensing optical system that condenses the signalbeam L1 and a second condensing optical system that condenses thereference beam L2. The shutter 140, the first focus variation unit 150,the first and second reflection members 132 and 134, a firstquarter-wave plate 165, the first objective lens 160 may constitute thefirst condensing optical system that condenses the signal beam L1. Thesecond focus variation unit 153, the third reflection member 136, asecond quarter-wave plate 175, and the second objective lens 170 mayconstitute a second condensing optical system for condensing thereference beam L2.

The first through third reflection members 132, 134, and 136 may beoptical members that fold the optical path to appropriately arrangeoptical devices. For example, the first through third reflection members132, 134, and 136 may be mirrors or total reflection prisms.

The shutter 140 may be an optical member that transmits or blocksincident light. In the method of recording holographic information usingthe shutter 140 according to the exemplary embodiment of the presentinvention, the shutter 140 may operate as follows. In a recordingoperation, the shutter 140 allows the signal beam L1 to passtherethrough during recording of the hologram mark 10 a and cuts off thesignal beam L1 during recording of the homogeneous mark 10 b. In areproduction operation, the shutter 140 may prevent the reproductionbeam L2′i from passing through the holographic data storage medium 190and traveling through a predetermined path to the optical path separator130 in an opposite direction to the signal beam L1.

The light L emitted by the light source 110 is not a completely linearlypolarized light and may partially include another linearly polarizedelement. Thus, in the reproduction operation, the light L traveling fromthe light source 110 may pass through the optical path separator 130 asthe reproduction beam L2′i, while part of the light L having anotherlinearly polarized element may be reflected by the optical pathseparator 130. Since the shutter 140 cuts off incident light in thereproduction operation as described above, the part of the light Lhaving another linearly polarized element may be cut off by the shutter140 and not allowed to travel toward the holographic data storage medium190.

When the method of recording holographic information according to theexemplary embodiment is embodied using an optical device other than theshutter 140, the shutter 140 may operate according to the otherfunctions described above. Specifically, the shutter 140 may be open todirectly transmit the signal beam L1 in a recording operation and may beclosed to prevent the reproduction beam L2′i from passing through theholographic data storage medium 190 and traveling through apredetermined path to the optical path separator 130 in an oppositedirection to the signal beam L1 in a reproduction operation.

The first and second focus variation units 150 and 153 vary focalpositions of the signal beam L1 and the reference beam L2 in theholographic data storage medium 190. For example, the first focusvariation unit 150 may include first and second relay lenses 151 and152. The first relay lens 151 may be mechanically driven to move in theoptical axis direction so as to vary the focal position of the signalbeam L1. Also, the second focus variation unit 153 may include third andfourth relay lenses 154 and 155. The third relay lens 154 may be drivento move in an optical axis direction so as to vary the focal position ofthe reference beam L2.

As described above, since the focal positions of the signal andreference beams L1 and L2 are varied by the first and second focusvariation units 150 and 153, holographic interference fringes, that is,the hologram mark 10 a, may be recorded on multiple layers of theholographic data storage medium 190.

The first and second quarter-wave plate 165 and 175 may convert linearlypolarized light incident to the holographic data storage medium 190 intocircularly polarized light and may convert circularly polarized lightreflected by the holographic data storage medium 190 into linearlypolarized light.

A recording operation of the holographic data recording/reproductionapparatus using the shutter 140 according the method of the presentinvention will now be described.

Referring to FIGS. 3A through 3D, and 6, the light source 110 emitslight L that is modulated according to data to be recorded in arecording operation. The polarization converter 125 converts the emittedlight L into light with an S-polarization element and a P-polarizationelement, and the optical path separator 130 separates the convertedlight into P-polarized light and S-polarized light. For brevity, it isassumed that the S-polarized light is reflected by the optical pathseparator 130 and becomes a signal beam L1, while the P-polarized lightis transmitted through the optical path separator 130 and becomes areference beam L2. The signal beam L1 is branched by the optical pathseparator 130, passes through the shutter 140, the first focus variationunit 150, and the first and second reflection members 132 and 134, andis condensed by the first objective lens 160 and incident to one side ofthe holographic data storage medium 190. Also, the reference beam L2 isbranched by the optical path separator 130, passes through the secondfocus variation unit 153, the third reflection member 136, and thesecond quarter-wave plate 175, and is condensed by the second objectivelens 170 and incident to the other side of the holographic data storagemedium 190.

The signal and reference beams L1 and L2 incident to respective sides ofthe holographic data storage medium 190 converge into a focus in theholographic data storage medium 190, and a hologram mark 10 a containingsingle-bit data due to a holographic interference fringe is recorded inthe focus.

After the hologram mark 10 a is recorded in a predetermined focalposition, when the signal beam L1 is cut off by the shutter 140, onlythe reference beam L2 is irradiated to the holographic data storagemedium 190 so that a homogeneous mark 10 b is recorded.

The operation of allowing the signal beam L1 to pass through the shutter140 to record the hologram mark 10 a and the operation of cutting offthe signal beam L1 using the shutter 140 to record the homogeneous mark10 b may be repeated, thereby forming a single recording layer on whichdata is recorded. As shown in FIG. 13, the operation of the shutter 140may be controlled by a holographic data recording/reproducing controller101 according to whether a hologram mark 10 a or a homogeneous mark 10 bis to be recorded. Also, while varying the foci of the condensed signaland reference beams L1 and L2 using the first and second focus variationunits 150 and 153, data may be recorded on a plurality of recordinglayers.

As described above, the recording method described with reference toFIGS. 3A through 3D may be modified. That is, the hologram mark 10 a isrecorded on a single layer, one of two beams may be cut off using theshutter 140 and the homogeneous mark 10 b may be recorded again betweenthe hologram marks 10 a.

The light source 110 may emit unmodulated light L in a reproductionoperation. When the light 110 is configured to emit only one linearlypolarized light or the polarization converter 125 is an active type,light passing through the polarization converter 125 is light polarizedlinearly in one direction. For brevity, it is assumed that light passingthrough the polarization converter 125 is P-polarized light.

P-polarized light, that is, a reproduction beam L2 i′, which istransmitted through the optical path separator 130, may travel throughthe second focus variation unit 153, the third reflection member 136,and the second quarter-wave plate 175 and is condensed by the secondobjective lens 170 and incident to the holographic data storage medium190. The incident reproduction beam L2 i′ may be reflected from therecording layer of the holographic data storage medium 190, and thereflected reproduction beam L2 r′ again passes through the secondobjective lens 170, the second quarter-wave plate 175, the thirdreflection member 136, the second focus variation unit 153 and isincident to the optical path separator 130. In this case, thepolarization direction of the reproduction beam L2 r′ reflected from theholographic data storage medium 190 is changed so that the reproductionbeam L2 r′ is reflected by the optical path separator 130 and incidentto the photodetector 180.

While it is exemplarily described above that the holographic datarecording/reproduction apparatus operates using the shutter 140, thepresent invention is not limited thereto. As stated above, a focuscontroller may be used instead of the shutter 140. Alternatively, whileemploying light beams emitted from a single light source, coherence maybe varied by changing driving conditions of the light source.Alternatively, two independent light sources may be adopted.

When the focus controller is used or when coherence is varied bychanging the driving conditions of the single light source, theholographic data recording/reproduction apparatus of FIG. 6 or otherholographic data recording/reproduction apparatus with a differentoptical construction may be employed. An example of an optical system ofa holographic data recording/reproduction apparatus using twoindependent light sources will be described later.

FIGS. 7A and 7B are schematic diagrams illustrating a method ofrecording holographic information using a focus controller according toan exemplary embodiment of the present invention. Specifically, in thepresent exemplary embodiment, the optical system of FIG. 6 is used, andthe first relay lens 151 of the first focus variation unit 150 disposedon the path of the signal beam L1 is used as the focus controller. Thus,FIGS. 7A and 7B illustrate a partial process of recording data bycontrolling a focal position of one of two incident light beamsaccording to the rotation of the holographic data storage medium 190.

Referring to FIG. 7A, when two light beams are incident from the lightsource 110 to respective sides of the holographic data storage medium190 and condensed into a focus, an interference fringe is generated.Since the refractive index of the holographic data storage medium 190varies in proportion to the intensity of the interference fringe, thehologram mark 10 a having a periodic variation in refractive index,which is illustrated with a fringe, is recorded in the holographic datastorage medium 190. Here, the focus is located on a virtual recordinglayer on which the hologram mark 10 a is to be formed.

Referring to FIG. 7B, when a focal position of one light beam is changedby driving the focus controller (i.e., the first relay lens 151 of thefirst focus variation unit 150), an interference fringe caused by twolight beams is only slightly formed at a recording position, so that thehomogeneous mark 10 b, which has an almost uniform refractive indexdistribution unlike the hologram mark 10 a, is recorded.

When controlling a position of the first relay lens 151 in the opticalaxis direction, the hologram mark 10 a with interference fringes and thehomogeneous mark 10 b with a uniform variation in refractive index arealternately formed in the holographic data storage medium 190. As shownin FIG. 13, the operation of the first focus variation unit 150 may becontrolled by a holographic data recording/reproducing controller 101according to whether a hologram mark 10 a or a homogeneous mark 10 b isto be recorded.

As described above, the recording method using the focus controlleraccording to FIG. 7A and 7B may be modified. In particular, an entirepredetermined recording layer may be recorded to provide the hologrammark 10 a, and after the hologram marks 10 a are recorded on apredetermined recording layer, the focal position of one of two beamsmay be changed such that an interference fringe caused by the two beamsis only slightly formed on the recording layer, thereby recording thehomogeneous mark 10 b between the hologram marks 10 a.

As a further alternative, when coherence is varied by changing drivingconditions (e.g., a driving frequency) of the light source 110 of FIG.6, the hologram marks 10 a and the homogeneous marks 10 b may bealternately formed.

FIGS. 8A and 8B show a lasing spectrum and the visibility of aninterference fringe, respectively, when a semiconductor LD functioningas the light source 110 oscillates in a single mode. FIGS. 9A and 9Bshow a lasing spectrum and the visibility of an interference fringe,respectively, when a semiconductor LD functioning as the light source110 oscillates in a multi-mode.

Referring to FIGS. 8A and 8B, it can be observed that when thesemiconductor LD oscillates in the single mode, interference fringes areuniformly formed. In contrast, referring to FIGS. 9A and 9B, it can beobserved that when the semiconductor LD oscillates in the multi-mode,interference fringes are formed at rare intervals. From this result, itcan be seen that interference fringes are not properly formed due topoor coherence during multi-mode oscillation.

FIGS. 10A and 10B are schematic diagrams illustrating a method ofrecording holographic information using the optical configuration ofFIG. 6 according to an exemplary embodiment of the present invention inwhich coherence is varied by changing the driving conditions of thelight source 110. Specifically, FIGS. 10A and 10B illustrate a partialprocess of alternately recording the hologram mark 10 a and thehomogeneous mark 10 b in the holographic data storage medium 190 byvarying the coherence of light emitted by the light source 110.

Referring to FIG. 10A, which illustrates a case where the light source110 is driven to emit light with good coherence, when two light beamsare incident from the light source 110 to both sides of the holographicdata storage medium 190 and condensed into a focus, an interferencefringe is generated. Since the refractive index of the holographic datastorage medium 190 varies in proportion to the intensity of theinterference fringe, the hologram mark 10 a having a periodic variationin refractive index, which is illustrated with a fringe, is recorded inthe holographic data storage medium 190. Here, the focus is located on avirtual recording layer on which the hologram mark 10 a is to be formed.

Referring to FIG. 10B, which illustrates a case where the light source110 is driven to emit light with poor coherence, even if two light beamsare incident from the light source 110 to both sides of the holographicdata storage medium 190 and condensed into a focus, an interferencefringe is not properly formed due to the poor coherence. As a result,the interference fringe is only slightly formed so that the homogeneousmark 10 b, which has an almost uniform refractive index distributionunlike the hologram mark 10 a, is recorded. As shown in FIG. 13, thedriving of the light source 110 may be controlled by a holographic datarecording/reproducing controller 101 according to whether a hologrammark 10 a or a homogeneous mark 10 b is to be recorded.

FIG. 11 is a diagram showing an example of an optical construction of aholographic data recording/reproduction apparatus using two independentlight sources to embody a method of recording holographic informationaccording to an exemplary embodiment of the present invention. FIGS. 12Aand 12B are schematic diagrams illustrating a method of recordingholographic information using two independent light sources according toan exemplary embodiment of the present invention.

An optical system of FIG. 11 is almost substantially the same as that ofFIG. 6 except that the holographic data recording/reproduction apparatusfurther includes a light source 210 that emits a light beam L3 used informing the homogeneous mark 10 b, a collimating lens 220 thatcollimates the light beam L3, and a beam splitter 230 that combines theoptical paths of the beams L3 and L1 to allow the light beam L3 totravel along the same optical path as the signal beam L1. FIG. 11illustrates the beam splitter 230 interposed between the shutter 140 andthe first focus variation unit 150, but the present invention is notlimited thereto. Alternatively, the light source 210, the collimatinglens 220, and the beam splitter 230 may be located to allow the lightbeam L3 to travel along an optical path of the reference beam L2.

Referring to FIG. 12A, during the recording of the hologram mark 10 a,only the light source 110 is driven to irradiate the signal beam L1 andthe reference beam L2 onto the holographic data storage medium 190 sothat the hologram mark 10 a can be recorded.

Referring to FIG. 12B, during the recording of the homogeneous mark 10b, the light source 110 is switched off and only the light source 210 isdriven to irradiate the light beam L3 onto the holographic data storagemedium 190 so that the homogeneous mark 10 b may be recorded.

As described above, the two independent light sources 110 and 210alternately driven so that the hologram mark 10 a and the homogeneousmark 10 b are alternately recorded. Alternatively, after the hologrammark 10 a is recorded on a single recording layer using the light source110, the light source 210 may be driven to record the homogeneous mark10 b between the hologram marks 10 a.

Hereinabove, it is described and illustrated that the shutter 140 or thefocus controller is used, coherence of light emitted by the light source110 is varied, or two independent light sources 110 and 210 are used inorder to embody a method of recording holographic information accordingto exemplary embodiments of the present invention. However, the presentinvention is not limited to the above-described methods, and anyrecording method that locates the homogeneous mark 10 b between thehologram marks 10 a may be applied.

Also, construction of an optical system of a holographic datarecording/reproduction apparatus that embodies a method of recordingholographic information according to exemplary embodiments of thepresent invention is not limited to FIGS. 6 and 11 and other variousconstructions may be applied to the optical system according to thepresent invention.

Furthermore, it is described above on the assumption that a method ofrecording holographic information according to exemplary embodiments ofthe present invention is applied to all recording layers. However, it isunnecessary from a practical standpoint to use the above-describedmethod on all layers because a recording layer disposed on the farthestrecording layer from an incident surface affects only a reproductionoperation. Accordingly, although it is possible to record a hologrammark and a homogeneous mark alternately on all recording layers, it isalso possible, if desired, to apply the method of the present inventiononly to recording layers that are positioned to affect other layers. Forexample, during multilayered recording of a holographic data storagemedium, it is possible to refrain from applying the method according toembodiments of the present invention to at least one recording layerthat affects less to an adjacent recording layer out of all recordinglayers. Thus, only a hologram mark may be recorded on the at least onerecording layer, while the hologram mark and a homogeneous mark may berecorded to be alternately located on the remaining recording layers.

In addition, it is described that a holographic datarecording/reproduction apparatus for embodying a method of recordingholographic information according to exemplary embodiments is atwo-sided incidence type, but the present invention is not limitedthereto. For example, a one-sided incidence type holographic datarecording/reproduction apparatus may be also used.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A method of recording holographic information, the method comprisingrecording a hologram mark and a homogeneous mark in a holographic datastorage medium having a volume to alternatively locate the hologram markand the homogeneous mark, the hologram mark having a varied refractiveindex distribution due to constructive/destructive interferences betweentwo light beams used to create the hologram mark and indicatinginformation, and the homogeneous mark having a refractive indexdistribution that is more uniform than the refractive index distributionof the hologram mark.
 2. The method of claim 1, wherein a thickness orrefractive index variation of the homogeneous mark is controlled suchthat a phase shift of a light beam passing through the homogeneous markis about the same as a phase shift of a light beam that passes throughthe hologram mark.
 3. The method of claim 1, wherein the homogeneousmark is recorded using a first light source and the hologram mark isrecorded using a second light source.
 4. The method of claim 1, whereinrecording of the homogeneous mark comprises cutting off one of the twolight beams used for recording the hologram mark such that thehomogeneous mark using the only the other one of the two light beams. 5.The method of claim 1, wherein recording of the homogeneous mark isperformed by changing a focal position of one of the two light beamsused for recording the hologram mark to reduce formation of aninterference fringe.
 6. The method of claim 1, wherein the hologram markand the homogeneous mark are recorded using the same light source whilevarying coherence by changing driving conditions of the light source,such that when the light source is driven to have a relatively greatercoherence, the hologram mark is recorded and when the light source isdriven to have a relatively lesser coherence, the homogeneous mark isrecorded.
 7. The method of claim 1, wherein the method includes forminga plurality of virtual recording layers in the holographic data storagemedium.
 8. The method of claim 7 wherein the recording of the hologrammark and the homogeneous mark is carried out on all of the virtualrecording layers.
 9. The method of claim 7, wherein in the forming of aplurality of virtual recording layers, only a hologram mark is recordedin at least one recording layer that affects less to an adjacentrecording layer out of all recording layers, and wherein a hologram markand a homogeneous mark are alternately recorded on the remainingrecording layers.
 10. A holographic data storage medium comprising: amaterial having a volume and having marks recorded within the volume,the marks comprising: a plurality of recorded hologram marksrepresenting data and having a varied refractive index distributionformed by constructive/destructive interferences between two lightbeams; and a plurality of recorded homogeneous marks being interspersedbetween the plurality of hologram marks and having a refractive indexdistribution that is more uniform than the refractive index distributionof the hologram marks.
 11. The holographic data storage medium of claim10, having a plurality of virtual recording layers within the volume,each recording layer including a plurality of the recorded hologrammarks and a plurality of the recorded homogeneous marks.
 12. Theholographic data storage medium of claim 11, wherein in each recordinglayer, a phase shift of a light beam passing through one of thehomogeneous marks is about the same as a phase shift of a light beamthat passes through one of the hologram marks.
 13. The holographic datastorage medium of claim 11, wherein at least one recording layerincludes holographic marks formed by multi-wavelength recording andwherein a mark reflection rate of the recording layer is uniform.
 14. Aholographic data recording/reproducing apparatus that records hologrammarks and homogenous marks on a holographic data storage medium, theholographic data recording/reproducing apparatus comprising: an opticalpickup unit comprising: a light source, an optical path separator thatdivides light from the light source into a first light beam and a secondlight beam, a first light path and a second light path along which thefirst light beam and the second light beam, respectively, are directedto be incident on the holographic data storage medium, a first focusvariation unit and a second focus variation unit that selectively focusthe first light beam and the second light beam, respectively, such thathologram marks each having a varied refractive index distribution due toconstructive/destructive interferences between the first light beam andthe second light beam are selectively formed on the hologram datastorage medium; and a shutter that selectively blocks the first lightpath; and a holographic data recording/reproducing controller thatcontrols one of the light source, the first focus variation unit and theshutter such that homogeneous marks each having a refractive indexdistribution that is more uniform than the refractive index distributionof the hologram mark are selectively formed on the hologram data storagemedium.
 15. The holographic data recording/reproducing apparatus ofclaim 14, wherein the holographic data recording/reproducing controllercontrols the shutter to selectively block the first light beam such thatthe homogeneous marks are formed by a reaction of only the second lightbeam with the holographic data storage medium.
 16. The holographic datarecording/reproducing apparatus of claim 14, wherein the holographicdata recording/reproducing controller controls the first focus variationunit to selectively change a focal position of the first light beam suchconstructive/destructive interferences between the first light beam andthe second light beam is reduced.
 17. The holographic datarecording/reproducing apparatus of claim 14, wherein the holographicdata recording/reproducing controller controls the light source toselectively change driving conditions of the light source to reducecoherence of the light from the light source.
 18. A holographic datarecording/reproducing apparatus that records hologram marks andhomogenous marks on a holographic data storage medium, the holographicdata recording/reproducing apparatus comprising: an optical pickup unitcomprising: a first light source, a first optical path separator thatdivides light from the first light source into a first light beam and asecond light beam, a first light path and a second light path alongwhich the first light beam and the second light beam, respectively, aredirected to be incident on the holographic data storage medium, a firstfocus variation unit and a second focus variation unit that selectivelyfocus the first light beam and the second light beam, respectively, suchthat hologram marks each having a varied refractive index distributiondue to constructive/destructive interferences between the first lightbeam and the second light beam are selectively formed on the hologramdata storage medium; a second light source; and a second optical pathseparator that directs light from the second light source to travelthrough the first light path or the second light path such that ahomogeneous mark is formed on the hologram data storage medium.